1. Field of the Invention
This invention relates to an active matrix type display device that has a light-emitting device such as an organic electroluminescent device (hereafter referred to as an organic EL device).
2. Description of the Related Art
Organic EL display devices using organic EL devices have been developed in recent years as display devices to replace CRT and LCD. An emphasis is laid on development of an active matrix type organic EL display device that uses a thin film transistor (hereafter referred to as a TFT) as a switching device to drive the organic EL device.
The active matrix type organic EL display device will be explained hereinafter, referring to the drawing. FIG. 9 is an equivalent circuit diagram of the organic EL display device. Only one pixel 210 is shown in FIG. 9 out of a plurality of pixels arrayed in a matrix form in a display panel of the organic EL display device. An N-channel type pixel selection TFT 213 is disposed around an intersection of a pixel selection signal line 211 extending in a row direction and a display signal line 212 extending in a column direction. A gate of the pixel selection TFT 213 is connected to the pixel selection signal line 211, while a drain of the pixel selection TFT 213 is connected to the display signal line 212. The pixel selection TFT 213 is turned on according to a high level of a pixel selection signal G, which is outputted from a vertical drive circuit 301 and applied to the pixel selection signal line 211. A display signal D is outputted from a horizontal drive circuit 302 to the display signal line 212.
A source of the pixel selection TFT 213 is connected to a gate of a P-channel type driver TFT 214. A source of the driver TFT 214 is connected to a power supply line 215 that supplies a positive power supply electric potential PVdd. A drain of the driver TFT 214 is connected to an anode of an organic EL device 216. A negative power supply electric potential CV is supplied to a cathode of the organic EL device 216.
A storage capacitor 218 is connected between the gate of the driver TFT 214 and a capacitor line 217. The capacitor line 217 is connected to a fixed electric potential. The storage capacitor 218 retains the display signal D applied to the gate of the driver TFT 214 through the pixel selection TFT 213 for one vertical period.
Next, operation of the organic EL display device described above will be explained. The pixel selection TFT 213 is turned on when the high level of the pixel selection signal G, which lasts for one horizontal period, is applied to the pixel selection line 211. Then the display signal D outputted to the display signal line 212 is applied to the gate of the driver TFT 214 through the pixel selection TFT 213 and retained by the storage capacitor 218. In other words, the display signal D is written into the pixel 210.
A conductance of the driver TFT 214 varies according to the display signal D applied to the gate of the driver TFT 214. When the driver TFT 214 is turned on, it provides the organic EL device 216 with an electric current corresponding to the conductance and the organic EL device 216 is driven to a brightness level corresponding to the electric current. On the other hand, when the driver TFT 214 is turned off accordingly to the display signal D supplied to its gate, the organic EL device 216 is extinguished because no electric current flows through the driver TFT 214. A desired image can be displayed on the entire display panel by performing the operation described above for all the rows of the display pixels 210 over one vertical period.
With the organic EL display device described above, however, there are problems of a variation in brightness on the display panel and a residual image of moving picture. A method to reduce the variation in brightness and the residual image of moving picture by controlling a light-emitting period of the organic EL device 216 using a scanning-related signal (the pixel selection signal G for example) of the vertical drive circuit 301 is known, as disclosed in Japanese Patent Application Publication No. 2002-175035. Assuming that a display area on the display panel is composed of pixels arrayed in a matrix with m rows and n columns and that the light-emitting period is a half of the vertical period for example, the organic EL device 216 is extinguished in synchronization with a rise of the pixel selection signal G on the (n/2)th row of the pixel selection signal line 211, according to the method.
However, the method disclosed in the Japanese Patent Application Publication No. 2002-175035 sets the light-emitting period using hardware. Thus, once the light-emitting period is set, changing the light-emitting period is not possible unless a connection of a wiring is physically modified. Modifying the connection of the wiring requires a change in a photomask for wiring, which causes problems of an additional cost for the photomask, an additional cost to manufacture a modified display panel and additional manufacturing time.